Preliminary report from the acoustic herring survey from north Shetland to Rona with R/V "G.O. Sars" 13 - 29 July 1981 (1.095Mb)

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WITH R/V "G.O. SARS" 13 - 29 July 1981.

by

A. Aglen and O.J. Østvedt

Institute of Marine Research, Bergen, Norway.

METHODS

Distribution and abundance of herring were estimated by aid of sonar mapping, echo integration and trawling. In addition data for "in situ" measurements of target strength were collected.

These data are not processed yet and have to await further ana- lyses. A Simrad EK 400/38 scientific echo sounder, a Simrad QD digital integrator and a NORD-lO computer were applied for echo

integration~

Settings and technical data:

Frequency:

Transducer:

10 log \l':

TVG and Gain:

38 kHz

30 x 30 cm, ceramic -21 dB

20 log R + 2u0.008·R -10 dB

Deviations from theoretical TVG: < 0.5 dB at the actual depths.

Threshold: 50 millivolt

Bandwith; 3.3 kHz, pulse length; 1.07 millisecond Source level + Voltage response: 134.9 dB,

measured by aid of a 60 mm copper sphere with

-33.7 dB target strength as described by Foote, Knudsen, Vestnes, Brede and Nielsen (1981).

The sonar was run at 1250 m basic range, 3° t i l t and auto training in 3° steps within 30° on each side.

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For sampling and identification of echo recordings three dif- ferent trawls were used; one pelagic trawl with maximum opening of 16 x 16 m, one pelagic trawl with maximum opening of 25 x 25 m and one bottom trawl with maximum opening of 20 x 6 m.

Hydrographic measurements were made with a CTD~zonde and zooplankton samples were taken with a Juday net.

For each trawl catch the species and size distribution was

established. Stomacs were preserved from cod, haddock, whiting, saithe and mackerel. Length, weight, sex, maturity stage and otholits were taken from 100 individuals from each significant catch of herring.

Integrator readings were allocated to four categories: Herring, bottom fish, 0-group fish and plankton. The allocation were mainly based on the appearance of the recordings, supported by

frequent trawling.

Average integrator values·

(M)

were calculated within 225 (n.mile) 2 squares. The herring biomass (B) within squares was estirnated

- o

84 -

as B = M

-I4-

L·225 (tennes). where L= average fish length (cm). The conversion factor ⁰

2~

⁴

-L

is based on an average target strength of -34 dB per kg for 24 cm herring and a -10 log

L

length dependence for average target strength per kg,

(Bailey et al. 1980).

RESULTS

Figure l shows the survey grid and stations. A 10 mile grid (10 n.mile between parallell track lines) was run through most of the area. This was increased to 5 mile grid in areas with good herring recordings.

In the Shetland area (East of 4°W) all the recordings identified as herring were dense schools typically extending from 20 to

60 m depth occurring clearly seperated from other fish recordings.

During a couple of hours of darkness the schools slightly

"loosened" but kept the school formation Most of the schools

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were found south-east of Foula and east of Somborough. Between first (15 - 20 July) and second (26 - 28 July) coverage of the area the schools had mooved slightly south-southwestward.

Smaller schools 5-10 m off bottom were recorded over wide

areas, especially between 2° and 4°W. None of these were identified as herring. Both pelagic and bottom trawl hauls gave

mainly Norway pout (O-gr), haddock and whiting (Table l).

South of Sule Skerry and Rona (West of 4°W) the herring was, however, exclusively found in small schools close to bottorn during daytime, while it raised about 20 m off bottom and scat- tered during the somewhat longer period of darkness in this area. The herring tended to be mixed with same haddock, whiting and gurnard (Table 2).

Figure 2 shows the distribution of echo sounder recordings and sonar contacts of herring along the track lines. The sonar was not run during the first 5 days of the cruise. The best sonar conditions were found in the area between Shetland and Orkney Islands where vertical temperature gradients were small. Schools detected in front of the vessel were aften observed avoiding to the one or the other side when the vessel approached.

The estirnated herring biornass within squares is shown in Figure 3.

rhe total estimate for the covered area west of 4°W is about 40 000 tonnes. In the Shetland area the first coverage gave about 100 000 tonnes and the second about 70 000 tonnes.

The samples from the Shetland area showed larger (and older) herring than the samples west of 4°W. Length distributions are shown in Figure 4 and Table 3 shows the age compositions.

The herring in the Shetland area was more mature than in the western area. The last trawl haul (July 28) south of Shetland gave 9% with running gonads.

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DISCUSSION

Because of the-observed avoidance, a significant amount of schools roay have been lost for echo integration. Too little data were collected to quantify this underestimation. More systematic use of sonar during later surveys may give an answer.

In the Shetland area where herring schools were clearly sepa- rated from other fish, the allocation of integrator values is believed to be precise. However, in the western area the allo- cations were more uncertain.

REFERENCES

BAILEY, R.S., AGLEN, A., CORTEN, A., DINER, N. and SIMMONDS, J.

1980. Report on the ICES-coordinated acoustic survey of Herring stocks in 1980. ICES C.M. 1980/H: 24.

FOOTE, K.G., KNUDSEN, H.P., VESTNES, G., BREDE, R. and NIELSEN, R.L. 1981. Improved calibration of hydroacoustic equipment with copper spheres .. ICES C.M. 1981/B: 20.

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Hau1 No.

Pelagic trawl

Position

240 60°35'N 00°17'W 242 59°55'N 00°52'W 243 59°55'N 00°39'W 244

245 247 248 250

59°56'!1 00°46'W 59°45'N 01°15'W 59°05'N 00°05'W 58°45'N 02°12'W 59°30'N 02°29.'W 251 5')⁰29 ,:N 02°29'i W 252 59°39'N 04°00'W 255 59°52'N 04°00'W 256 59°43'N 02°33'W 257 60°25'N 00°28'W 258 60°58'N 00°49'W 261 60°52'N 01°12'W 262 6l⁰03'N Ol⁰13'W 265 60°20'N 01°57'W 266 59°5l'N Ol⁰3l'W 28G 59°47'N 01°52'W 7.88 59°53'N 00°57'W

Bot: tom

trawl

239 60°45'N 00°16¹W 241 60°15'N 00°02'W 246 59°25'N 01°17'W 249 59°29'N 01°27'W 253 59°40'N 03°01'W 254 S9°45'N 02°30'W 259 60°47'N Ol⁰43'W 2GO 60°44'N 02°13'W 263 Gl⁰08'N Ol⁰18'W

~b4 G0°2l'N 01°59'W 284 59°47'N 02°29'W 285 60°00'N 02°10'W 287 59°47'N Ol⁰30'W

Date

14/7 15/7 15/7 15/7 16/7 17/7 17/7 18/7 18/7 18/7 18/7 19/7 20/7 20/7 21/7 22/7 23/7 23/7 28/7 28/7

14/7 1.5/7 16/7 17/7 18/7 18/7 21/7 21/7 22/7 22/7 27/7 27/7 28/7

Ho ur (GM'l')

2250 1355 1610 2335 0250 0100 0900 0250 0400 0940 2350 2310 1725 2215 2135 0035 0040 013U 0140 1430

1750 0650 1330 1800 1345 1730 0850 1315 0345 1435 1055 1830 0835

Duration (min.)

20 35 50 55 45 20 60 60 25 20 20 20 30 20 15 30 45 20 50 30

60 60 60 60 45 30 40 30 30 15 60 30 30

Catches in kg Herring Others

40.3 2.0

4000.-

EiOO.-

34.1

6.5 No catch

6.3 3.3 0.6 16.0·

101.5 35.3

o. 3.

3.3 No catch

+ + 57.6 50.0 202.2

2.0 11.9

+

195.5 156.- 204.6

62.7 108.5 26.7 65.4 144.5 172.1 22.2 54.6

?L 3 20.6

Predorninant species

Norway pout (O-gr).

Haddock Herr ing Herr ing

Norway pout (O-gr) Whiting

Whiting Whiting Gurnard

Herr ing

Norway pout (O-gr) Norway pout (O-gr) Norway pout (O-gr) Norway pout (O-gr) Herr ing

Whiting Herr ing Haddock (O-gr)

Haddock Haddock Haddock Whiting C od Haddock Haddock Norway pout Norway pout

Lesser-spott.ed dogfish Lesser-spotted dogfish Haddock

Haddock

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Tab le 2. Trawl catches made by G.O.SARS west of 4°N.

Ha ul Position Date Ho ur Dura ti on Catches in kg

No. (GMT) - (min.) . Herring Others Predqminant spe eies

Pelagic trawl

267 59°10'N 04°00'W 23/7 1155 30 0.3 ,Haddock

269 58°40'N 05°55'W 23/7 2300 40 0.2 12.4 Gurnard

270 58°40'N 06°00'W 24/7 0025 20 '450.- 18.5 Herr ing 272 58°~.1' N 05°50' W 24/7 1010 60 4.3 2.6 Herr ing

274 58°46'N 05°48'W 25/7 0320 30 2 .o· Herr ing

275 58°46'N 04°54'w' 25/7 0650

::'",

50 2-.4 Haddock

277 S8°45'N 04°55'W 25/7 llOS 45 0.4 Haddock

278 59°18' N 04° 49 'w: 25/7 1535 30 No· catch

279 58°S8' N 04\!';)'20' w 26/7 0445 llO O.l· 16.1 Norway pout (O-gr)

280 59°15'N 04°14'W 26/7 1510 30 6.5 Whiting

282 59°30'N 04°43'W 26/7 2155 15 10.9 Norway pout (O-gr)

283 59°35'N 04°35'W 27/7 0100 30 lO Norway pout (O-gr)

Bot tom trawl

268 58°42'N 06°05'W 23/7 2200 15 32.0 137.8 Haddock

271 58°5l'N 05°40'W 24/7 0825 45 50.1 Haddock

273 59°20'N 06°20'W 24/7 1810 60 441.5 Sheppy argentine

276 58C>46'N 04°52'W 25/7 0900 60 133.7 Haddock

281 59°26'N 04°30'W 26/7 1840 60 91.2 Whiting

Table 3 .. Age composition (%) in samples taken by "G.O. Sars"

July 1981 ..

Year Class 79 78 77 76 75 74 73 <73 No. of o to-

Age (winter liths

rings) l 2 3 4 5 6 7 >7

East of 4°W Oø4 0.4 3 .. 8 33 .. 6 10 .. 9 7.6 41.2 2.1 238 West of 4°W o 12 .. 5 47.9 26.7 4 .. 2 Oe4 8.3

o

240

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b.

70 ,. _{' l}_{t : . .}_r_{~ .~} _L

z 1 6 2

o ³

60 0u r \⁷CY ·~i r ^j

50 40

(j ar:(: .. i ::·; t {·~ ( ^i.()^{l l}^~J

30 20 10 00

o ^~,

o Sa.r. s '' l : Hydroqrapt1

' J

station, 2: Pelagic trawl, 3: Bottorn trawl 4:

Zooplankton sample. Black symbols rneans catches of herring.

a. July 14 - 26 b. July 26 - 28

t.C

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a.

b.

- 1 o 2

Figure 2. Echo sounder recordings (l) and sonar contacts (2) of herring, "GeO .. Sars".

a. July 14 - 26 be July 26 ^~ 28

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Figure 3. Estimated biomass (1000 tonnes.) within squares, "G.O.

Sars¹¹ ^•

a. Ju1y 14 - 26 b. July 26 - 28

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a.

b,

30 N= 518

O/O

20

10

18

22

26 30 34

38 cm

N

O/O

cm

Figure 4 .. Length distribution of herring, "G .. O .. Sars".

(N

=

a ..

b ..

number measured) East of 4°W

West of 4°W